1. Field of the Invention
The present invention relates to a piezoelectric resonator, a process for the fabrication thereof, its use as a sensor element, which, implemented in a through-flow cell, is integrable in a measurement system for the determination of the concentration of a substance contained in a liquid and/or for the determination of the physical properties of the liquid. The piezoelectric resonator is designed plane and is provided, on its surface, with electric contact areas for electrodes and counter electrodes, which is connectable to a signal source as well as to a measurement device. For measuring, the piezoelectric resonator is brought into contact with the to-be-examined liquid on one side, and the resonator responds to the accumulation of the mass of the to-be-detected substance or to a change in the physical properties of the liquid by altering its resonance frequency and/or oscillation amplitude.
2. Description of the Prior Art
Usually, piezoelectric resonators are provided with small oscillator quartz plates, which provide conductive contact areas for the integration and the electric contacting to an electric oscillation circuit, via which connecting an alternating voltage, which resonates typically for the small plates, can be applied to the small oscillator quartz plates. However, due to the structure of the small plates, which can meanwhile be fabricated in a filigree manner, the resonating behavior of the resonator may be impaired, i.e. by mechanical tension in the small plates caused by the holding carriers or by contacting with electric lines.
An example of a small oscillator quartz plate that is fixed in a holding carrier is described in DE 34 46 612 A1. The small oscillator quartz plate 8 is provided with electrodes 301 deposited on its surface and is spatially fixed between two holding clamps 201 and electrically contacted. What is disadvantageous in this setup, however, is that even with the least exerted force, which can act on the two parts 7 which are spaced apart, can generate mechanical tension in the small plate 8 glued between parts 7, thereby influencing the resonance behavior of small plate 8.
Among other things, piezoelectric resonators are used as active sensor elements, for example, for detecting a substance in a medium, for instance, for measuring the concentration of the substance in a liquid. In addition to the aforementioned problem of ensuring tension-free holding of a small oscillator quartz plate, when working with liquids, the oscillator quartz to be electrically contacted has also to be insulated liquid-tight against the to-be-examined liquid in order to prevent electric short circuits. Such a sensor is described in DE 40 13 665. This known sensor provides a small oscillator quartz plate 4, which is clamped on both sides between two silicon seals and, in addition, is contacted to conductive adhesive substances. However, the use of conductive adhesives has the consequence that the electric contact cannot be disconnected, which, for instance, makes replacing the small oscillator quartz plate impossible or at least requires great manual skill. Moreover, the silicon seals surrounding the small oscillator plates from all sides have to be made with great precision in order to prevent deformations in the small plate.
A similar setup for measuring the concentration of certain substances dissolved in a liquid using a small oscillator quartz plate is described in the article by W. Stockl and R. Schumacher, "In Situ Microweighing at the Junction Metal/Electrode", Berichte der Bunsengesellschaft physikalische Chemie 91 (1987, pp. 345-349. The small oscillator quartz plate is electrically contacted on both sides at its edge areas. By means of a suited liquid sealing at one side of the small oscillator quartz plate, the to-be-examined liquid comes into surface contact with the small plate so that the resonance behavior of the small plate alters accordingly and measuring can be conducted in the known manner. Similarly operating sensors using piezoelectric resonators for immunoanalysis are known from the articles by K. Davis, T. Leary, "Continuous Liquid-Phase Piezoelectric Biosensor of Immunoassays", Anal. Chem. 1989, No. 61, pp. 1227-1230 and by M. Thomson et al., "The Potential of the Bulk Acoustic Wave Device as a Liquid-Phase Immunosensor", IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, volume USFC 34, NO. 2, March 1987, pp. 127-135.
All the known sensors for measuring concentrations of a substance in liquids and for determining the physical properties of liquids operating on the basis of piezoelectric resonators have in brief the following drawbacks:
The small oscillator quartz plates are glued or clamped in a holding means, which can impair the resonance behavior of the resonator itself due to mechanical tensions. Moreover, glued and clamped electric contacts on the surface of small oscillator quartz plates are not totally reliable, in particular, when employed for measuring concentrations of substances in liquids. Precautions must be taken in order to avoid short circuits. A stable and uniform quality of the contacting is not possible by this means.
In all the known cases, the piezoelectric resonators are provided with contact areas, which are to be connected to an electrical oscillation circuit and respectively to a measuring system for electrical contacting, on both its front side as well as on its back side. The integration of the piezoelectric resonator in a casing with respective electric contacting is difficult and time consuming. Good stable quality of the electric properties, in particular, when using a piezoelectric resonator in a holding means, which permits bringing the small oscillator plate into contact with a to-be-examined liquid on one side, is impossible.
Holding means of this type are also called flow-through cells, which are a unit, in which the already electrically contacted small oscillator quartz plate is connected to the connecting electrodes. Furthermore, defined supply and drain channels are provided via which the to-be-examined liquid can be selectively supplied to the piezoelectric resonator and drained again. Such a type of flow-through cell is shown, for instance in FIG. 4 in M. Thomson's article cited above.
For flow-through cells of this type, the user needs much time and manual skill in replacing the cell. In particular, in the field of biosensors, replacement often becomes necessary, because each different to-be-detected substance requires an especially prepared flow-through cell. The work involved in this replacing hinders wide commercial acceptance of piezoelectric resonators.